If You Drop It, Should You Eat It? Scientists Weigh In on the 5-Second Rule
September 2, 2003
September 2, 2003

URBANA--High-school student Jillian Clarke investigated the scientific validity of the "5-second rule" during her apprenticeship in Hans Blaschek's University of Illinois lab this summer. You know the rule: If food falls to the floor and it's in contact with the floor for fewer than 5 seconds, it's safe to pick it up and eat it.

According to Clarke, a senior at the Chicago High School for Agricultural Sciences, the 5-second rule dates back to the time of Genghis Khan, who first determined how long it was safe for food to remain on a floor when dropped there. Khan had slightly lower standards, however; he specified 12 hours, more or less.

Among Clarke's findings:

--Seventy percent of women and 56 percent of men are familiar with the 5-second rule, and most use it to make decisions about tasty treats that slip through their fingers.

--University floors are remarkably clean from a microbial standpoint.

--Women are more likely than men to eat food that's been on the floor.

--Cookies and candy are much more likely to be picked up and eaten than cauliflower or broccoli.

--And, if you drop your food on a floor that does contain microorganisms, the food can be contaminated in 5 seconds or less.

A participant in the College of Agricultural, Consumer and Environmental Sciences' summer Research Apprentice Program, Clarke began by swabbing 1-inch squares of floors in a variety of locations on the U of I campus, including floors in high-traffic areas.

"We were shocked," said Meredith Agle, a Ph.D. candidate in Blaschek's food microbiology labs, who helped Clarke with the experiment. "We didn't even find a countable number of bacteria on the floor. We thought we might have made a mistake, so we tried again with the same result.

"Then we went back to look for spore-forming organisms, such as Bacillus, something that would resist dry conditions, but we couldn't find any spores either," Agle said.

Clarke then purchased smooth and rough 2-inch tiles from the hardware store so she could experiment with different surfaces and a good supply of gummy bears and fudge-striped cookies from the grocery store. Clarke's survey showed that people were more likely to retrieve cookies or candy because they value them more highly. Cookies and candy also have low levels of naturally occurring microflora, unlike fresh vegetables, meat, or cheese.

The next step was sterilizing the tiles and inoculating them with E. coli, then placing 25 grams of cookies or gummies on the tiles for 5 seconds. In all cases, E. coli was transferred from the tile to the food, demonstrating that microorganisms can be transferred from ceramic tile to food in 5 seconds or less. More E. coli were transferred to gummy bears from smooth tiles than from rough tiles.

To examine the surfaces of the tiles and the food, Clarke enlisted the help of Chas Conway at the U of I's Beckman Institute for Advanced Science and Technology. He showed her how to use scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM) to look at the tiles, cookies, and bears in minute detail.

Because SEM preparation causes dehydration in the sample, SEM gummy bears were especially shriveled and shrunken, resulting in an inaccurate image. Clarke was able to obtain a much more realistic image of the gummy bear surfaces with ESEM, which led her to conclude that ESEM technology, which allows foods to be imaged in their natural, hydrated state, is a more useful tool for examining such specimens.

The College of ACES' Research Apprentice Program is an intensive seven-week laboratory and academic summer program that provides hands-on science experience for talented high school juniors and seniors who are interested in careers in the food, agricultural, and environmental sciences.